Palladium-spot leadframes for high adhesion semiconductor devices and method of fabrication

ABSTRACT

A leadframe for use in the assembly of integrated circuit chips comprising a base metal structure having an adherent layer of nickel covering said base metal; an adherent film of palladium on said nickel layer; and an adherent layer of palladium on said palladium film, selectively covering areas of said leadframe suitable for bonding wire attachment and solder attachment.

This is a continuation of application Ser. No. 09/733,718 filed Dec. 8,2000, which claims priority of application Ser. No. 60/170,248 filed onDec. 10, 1999, now U.S. Pat. No. 6,953,986.

FIELD OF THE INVENTION

The present invention is related in general to the field ofsemiconductor devices and processes and more specifically to thematerials and fabrication of leadframes for integrated circuit devices.

DESCRIPTION OF THE RELATED ART

The leadframe for semiconductor devices was invented (U.S. Pat. No.3,716,764 and U.S. Pat. No. 4,034,027) to serve several needs ofsemiconductor devices and their operation simultaneously: First of all,the leadframe provides a stable support pad for firmly positioning thesemiconductor chip, usually an integrated circuit (IC) chip. Since theleadframe including the pads is made of electrically conductivematerial, the pad may be biased, when needed, to any electricalpotential required by the network involving the semiconductor device,especially the ground potential.

Secondly, the leadframe offers a plurality of conductive segments tobring various electrical conductors into close proximity of the chip.The remaining gap between the (“inner”) tip of the segments and theconductor pads on the IC surface are typically bridged by thin metallicwires, individually bonded to the IC contact pads and the leadframesegments. Obviously, the technique of wire bonding implies that reliablewelds can be formed at the (inner) segment tips.

Thirdly, the ends of the lead segment remote from the IC chip (“outer”tips) need to be electrically and mechanically connected to “otherparts” or the “outside world”, for instance to assembly printed circuitboards. In the overwhelming majority of electronic applications, thisattachment is performed by soldering. Obviously, the technique ofsoldering implies that reliable wetting and solder contact can beperformed at the (outer) segment tips.

It has been common practice to manufacture single piece leadframes fromthin (about 120 to 250 μm) sheets of metal. For reasons of easymanufacturing, the commonly selected starting metals are copper, copperalloys, iron-nickel alloys (for instance the so-called “Alloy 42”), andinvar. The desired shape of the leadframe is etched or stamped from theoriginal sheet. In this manner, an individual segment of the leadframetakes the form of a thin metallic strip with its particular geometricshape determined by the design. For most purposes, the length of atypical segment is considerably longer than its width.

In the European patent No. 0 335 608 B1, issued 14 Jun. 1995 (Abbott,“Leadframe with Reduced Corrosion”), a palladium-plated leadframe isintroduced which is not subject to corrosion due to galvanic potentialforces aiding the migration of the base metal ions to the top surfacewhere they will form corrosion products. The. patent describes asequence of layers consisting of nickel (over the base metal),palladium/nickel alloy, nickel, and palladium (outermost). Thistechnology has been widely accepted by the semiconductor industry.

After assembly on the leadframe, most ICs are encapsulated, commonly byplastic material in a molding process. It is essential that the moldingcompound, usually an epoxy-based thermoset compound, has good adhesionto the leadframe and the device parts it encapsulates. Palladium,described above as the outermost layer of the leadframe, offersexcellent adhesion to molding compounds.

Unfortunately, palladium is expensive; its price climbed in the lastdecade from about one third of the gold price to about 20% higher thangold. Cost reduction pressures in semiconductor manufacturing haveinitiated efforts to reduce the thickness of the palladium layersemployed to about one third of its previous thickness. At this thinness,palladium does not prevent oxidation of the underlying nickel which willinhibit its solderability (while maintaining bondability). A methodintroduced in semiconductor manufacturing uses a thin layer of gold onthe palladium surface to prevent oxidation. One related example isdescribed in U.S. Pat. No. 5,859,471, issued on Jan. 12, 1999 (Kuraishiet al., “Semiconductor Device having TAB Tape Leadframe with ReinforcedOuter Leads”).

In these methods, however, are also expensive and severely inhibit theadhesion of the leadframe segments to molding compounds and thus riskdelamination in thermomechnical stress testing. Furthermore, any platingof the complete leadframe makes it difficult to decide by visualinspection whether a leadframe has a surface different from nickel ornot. Such standard simple inspection, however, is highly desirable asmanufacturing practice.

Last but not least, any plating or leadframe surface treatment has tomaintain or promote solderability of the outer tips of the leadsegments.

In U.S. Patent Application No. 60/138,070, filed on 8 Jun. 1999, towhich the present invention is related, a fabrication process forpalladium layers of reduced thickness is described (combined with aprocess for plating solder layers). However, in this method still toomuch palladium is consumed so that not enough cost reduction isachieved.

An urgent need has therefore arisen for a low-cost, reliable massproduction method for a leadframe having reduced palladium layerthickness combined with solderablility, bondability, adhesion capabilityto molding compounds, and visual inspection contrasts. The leadframe andits method of fabrication should be flexible enough to be applied fordifferent semiconductor product families and a wide spectrum of designand assembly variations, and should achieve improvements toward thegoals of improved process yields and device reliability. Preferably,these innovations should be accomplished using the installed equipmentbase so that no investment in new manufacturing machines is needed.

SUMMARY OF THE INVENTION

According to the present invention for a semiconductor integratedcircuit (IC) leadframe, a base metal having a plated layer of nickelfully covering the base metal has a plated layer of palladium on thenickel layer selectively covering areas of the leadframe suitable forbonding wire attachment and solder attachment. Further, a plated thinfilm of palladium insures excellent leadframe adhesion to theencapsulating molding compound by allowing some nickel and nickel-oxideon the leadframe surface.

The present invention is related to high density ICs, especially thosehaving high numbers of inputs/outputs, or contact pads, and also todevices in packages requiring surface mount in printed circuit boardassembly. These ICs can be found in many semiconductor device familiessuch as standard linear and logic products, digital signal processors,microprocessors, digital and analog devices, high frequency and highpower devices, and both large and small area chip categories. Theinvention represents a significant cost reduction of the semiconductorpackages, especially the plastic molded packages, compared to theconventional copper-based palladium-plated leadframes.

It is an aspect of the present invention to provide a technology forreducing the amount of costly noble metal, especially palladium, layerswhile simultaneously maintaining the bondability and solderability ofthe leadframe and improving its reliable adhesion to plastic moldingcompounds.

Another aspect of the invention is to apply the noble metal, such aspalladium, in different thicknesses on the leadframe segments by platingin two steps.

Another aspect of the invention is to reach these goals without the costof equipment changes and new capital investment, by using the installedfabrication equipment base.

Another aspect of the invention is to produce leadframes so thatestablished wire bonding processes can continue unchanged, and thatestablished board attachment process can continue unchanged.

Another aspect of the present invention is to introduce a manufacturingquality check based on a simple, low-cost visual inspection. This checkinsures the selection of the correct leadframe and its appropriatepreparation before releasing it into the assembly process flow.

Another aspect of the invention is to introduce a palladium spot platingtechnology with provides loose tolerance for the spot boundaries, thussimplifying leadframe manufacturing and lowering fabrication cost.

These aspects have been achieved by the teachings of the inventionconcerning masking and deposition methods suitable for mass production.Various modifications of leadframe preparations have been successfullyemployed.

In the first embodiment of the invention, applicable especially forthrough-hole leadframes, the spot-plated palladium is deposited on thetop and bottom surfaces of the external lead segment ends (and, ofcourse, on the wire bonding areas of the internal lead ends).

In the second embodiment of the invention, applicable especially forsurface-mount leadframes, the spot-plated palladium is deposited only onthe bottom surface of the external lead segment ends involved in solderattachment to motherboards (and on the wire bonding areas of theinternal lead ends).

For all embodiments, solderability, bondability, adhesion to plastics,and corrosion insensitivity are demonstrated. In this invention, thesurface of the leadframe has nickel, nickel oxide and palladiumcharacter. This combination hinders copper creep corrosion, as comparedto a surface with pure palladium.

Leadframes prepared according to the invention can be successfully usedin surface mount technologies based on bending the package leadsegments.

The technical advances represented by the invention, as well as theaspects thereof, will become apparent from the following description ofthe preferred embodiments of the invention, when considered inconjunction with the accompanying drawings and the novel features setforth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified top view of a leadframe unit for through-holesemiconductor devices, delineating the masking needed in the fabricationmethod of the first embodiment of the invention.

FIG. 2 is a schematic cross sectional view of a portion of a leadframe(as shown in FIG. 1) made according to the first embodiment of theinvention.

FIG. 3A illustrates schematically portions of the plating apparatus usedin fabricating leadframes according to the invention.

FIG. 3B illustrates schematically detail of the wheel used in theplating apparatus of FIG. 3A.

FIG. 4 is a schematic and simplified cross sectional view of a packaged,surface-mounted semiconductor device having a leadframe according to thesecond embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is related to the assembly of semiconductor ICs onleadframes, including wire bonding interconnection, and their finalencapsulation, the sequential construction of these leadframes usingdeposited layers of various metals, and the process of fabricating theseleadframes so that they offer quality-related visual inspection andreliable solder attachment to substrates.

The invention reduces the cost of leadframes while the leadframefunctions are maximized. The invention best applies to any leadframe andany substrate used in semiconductor technology which exhibit thefollowing design features: Usually, a chip mount pad for support of theIC chip surrounded by lead segments, each having a first end inproximity of the chip pad, and a second end remote from the chip pad.The invention thus applies to semiconductor package types such as PDIPs,SOICs, QFPs, SSOPs, TQFPs, TSSOPs and TVSOPs.

As defined herein, the starting material of the leadframe is called the“base metal”, indicating the type of metal. Consequently, the term “basemetal” is not to be construed in an electrochemical sense (as inopposition to ‘noble metal’) or in a structural sense. The base metal ofleadframes is typically copper or copper alloys. Other choices comprisebrass, aluminum, iron-nickel alloys (“Alloy 42”), and invar.

Leadframe segments have to satisfy five needs in semiconductor assembly:

-   -   1) Leadframes have to comprise segment ends remote from the chip        mount pad (“outer segments”) for solder attachment to other        parts;    -   2) leadframes have to comprise segment ends near the chip mount        pad (“inner segments”) for bond attachments to wire        interconnections;    -   3) leadframes have to comprise outer segments ductile for        forming and bending the segments;    -   4) leadframe surfaces have to comprise adhesion to molding        compounds, and    -   5) leadframe segments have to comprise insensitivity to        corrosion.

According to the teachings of this invention, Need 1) is satisfied bydepositing a layer of nickel, fully covering the leadframe base metal,and then selectively plating a layer of palladium where a solder jointhas to be made. In the first embodiment of the invention, applicableespecially to leadframes with pin-shaped outer segment ends (forthrough-hole assembly of devices), palladium is plated on both surfacesof the segment ends. In the second embodiment of the invention,applicable especially to leadframes with gull-wing or J-shaped outersegment ends (for surface mount assembly of devices), palladium isplated only on the surface of the segment ends facing the assemblyboard.

This invention provides the option to retain the palladium thicknesswhere it is needed for solder attachment purposes, but reduces it inother places. For example, for surface-mount devices with gull-wingshaped leads, the outer surfaces may retain 70–80 nm palladium, butexhibit only 20–30 nm on the inner surfaces. This is achieved by atwo-step plating process.

The invention satisfies Need 2) by first plating the nickel layer, fullycovering the leadframe base metal as outlined above, and then plating athin layer of palladium onto the nickel layer, selectively coveringareas of the leadframe which are intended for bonding wire attachment(and chip attachment). For palladium, a thin layer is sufficient forreliable bonding wire attachment (stitch bonds, ball bonds, or wedgebonds).

The invention satisfies Need 3) by the selection of thickness andstructure of the nickel layer employed to fulfill Need 1). Thickness anddeposition method of the nickel layer have to be selected such that thelayer insures ductility and enables the bending and forming of the outerlead segments.

The invention satisfies Need 4) by depositing a thin film of palladiumon the whole nickel surface of the leadframe concurrently with theplating process employed for fulfilling need 2). The surface thusprovides for the formation of nickel oxide in amounts aiming to maximizeadhesion of the leadframe to thermoset molding compounds and otherencapsulation materials.

The invention satisfies Need 5) by the sequence of layers deposited overthe copper base. The improved corrosion insensitivity in mixed flowinggas corrosion tests is due to the leadframe surface achieved by theinvention, which has nickel, nickel oxide and palladium character. Thissurface hinders copper creep corrosion (as compared to surfaces withpure palladium), which is a function of the nobility of the exposedsurface.

According to the invention, the palladium layer required to satisfyNeeds 1) and 2) is deposited by a selective plating process. Theselective characteristic of the palladium deposition is achieved by atemporary masking step, which leaves only those leadframe portionsexposed which are intended to receive the palladium layer. An example ofthe extent of such masking is depicted in FIG. 1, which shows a single(interdigitated) leadframe unit from a continuous strip, for a typicalsemiconductor Dual-In-Line device. The temporarily masked parts of theleadframe unit are shaded and the exposed parts are unshaded. Theunmasked parts include outer leadframe segments 111, further portions112 a of the inner lead segments 112, and the chip mount pad 130.

As defined herein, each lead segment 110 has a first end 112 a near thechip mount pad 113 and a second end 111 remote from chip mount pad 113.For the dual-in-line leadframe of FIG. 1, the second segment end 111represents the external lead (for some surface-mount leadframes,however, the segment functions may be designed as two distinct parts).In the example of FIG. 1, the leadframe unit has 16 lead segments 110,since it is designed for a plastic 16-pin dual-in-line package (PDIP).In summary, the exposed parts which receive the palladium plating,include the first ends 112 a of lead segments 110, the second ends 111of the lead segments, and the chip mount pad 130.

After the selective plating of the palladium layer, a cross sectionalong line “A—A” in FIG. 1 results in the schematic view generallydesignated 200 in FIG. 2, exaggerated in the vertical for clarity. Thecopper or copper alloy base sheet 201 has a preferred thickness in therange from 100 to 300 μm; thinner sheets are possible. The ductility inthis thickness range provides the 5 to 15% elongation needed in anysegment bending and forming operation. The leadframe is stamped oretched from the starting metal sheet.

The stamped or etched leadframe is first immersed in an alkalinepreclean solution at 20 to 90° C. for few seconds up to 3 minutes. Oils,grease, soil, dirt and other contamination are thereby removed. Afterrinsing, the leadframe is next immersed in an acid activation bath atroom temperature for few seconds up to 5 minutes. The bath consists of asolution of sulfuric acid, hydrochlorid acid, or other acid solution,preferably at about 30 to 60 g/l concentration. This solution removescopper oxide and leaves the metallic copper oxide surface in anactivated state, ready to accept the deposition of metallic nickel.

The nickel layers 202 are electroplated to a thickness in the rangebetween about 50 and 150 nm. The next deposited layers 203 are an alloybetween nickel and a noble metal selected from a group consisting ofpalladium, rhodium, gold silver, and platinum. The preferred choice ispalladium, with 60 to 80% palladium. The alloy layer is deposited byelectroplating and between about 25 and 150 nm thick; it should becoherent since its main purpose is corrosion protection.

The important layers 204 are electroplated nickel, deposited preferablyfor a thickness of about 0.5 to 3 μm. This nickel layer has to beductile in order to be malleable in any leadframe segment bending andforming process. Further, the nickel surface has to be wettable in thesoldering process, so that solder alloys or conductive adhesives can beused successfully.

The overall thickness of the two nickel layers and the nickel alloylayer is in the range of about 650 to 4000 nm.

The next deposited layers of the embodiment in FIG. 2 are the layers 205and 206, comprising an electroplated noble metal selected from a groupconsisting of palladium, rhodium, gold and silver. Layers 205 and 206are made of the same materials, since they are deposited in the sameprocess step (see below under “Wheel System”). The preferred embodimentis palladium. According to the invention, layer 105 is a thin filmbetween about 1 and 5 nm thick, when palladium is chosen. At thisthinness, only very little palladium material is consumed, and somenickel from underlying layer 204 can diffuse through the palladium andoxidize at the surface to nickel oxide. Consequently, the surfaceassumes a combined nickel/nickel oxide/palladium character and thenobility of the exposed surface is reduced. This fact, in turn,diminishes the copper creep corrosion of the leadframe, which is afunction of the nobility of the exposed surface.

Furthermore, it is pivotal for the present invention that the describedpalladium film provides for excellent adhesion to thermoplastic moldingcompounds—an attribute crucial for avoiding package delamination andrelated degradations such as the infamous “popcorn effect”.

Layers 206 are between about 70 and 90 nm thick. According to theinvention, they are deposited onto the leadframe surfaces not masked inthe masking step described above. In FIG. 2, palladium layer portionsare deposited in the areas of the “remote” segments ends onto the firstleadframe surface 220 to form layers 206 a, and onto the secondleadframe surface 230 to form layers 206 b. Both palladium layerportions provide the precondition for successful solder attachment.Further, palladium layer portions are deposited in the areas of the“near” segment ends onto the first leadframe surface 220 to form layers206 c. These palladium portions provide the precondition for successfulbond wire attachment (stitch bonds, ball bonds, and wedge bonds).

It should be noted that in the thickness range from 70 to 90 nm,palladium provides a visual distinction between the plated areas and theadjacent thin palladium film surfaces. This contrast between covered andnot-covered areas can readily be noticed by the unaided eye and is,therefore, well suited for automated visual inspection in manufacturingprocess control, contributing to product quality assurance.

There are several methods to selectively deposit metals from solutiononto a continuous strip. For high volume production of leadframes,continuous strip or reel-to-reel plating is advantageous and commonpractice. Based on the loose tolerance acceptable for the boundaries ofthe palladium plating on the first ends of the lead segments, thepreferred deposition method for the present invention is the so-called“wheel system”. The process steps are as follows.

Wheel System

-   -   Material is moved over a large diameter wheel with apertures in        it to allow solution flow to material;    -   apertures define the locations for plating; index pins engage        the pilot holes (designated 37 in FIG. 3) in the leadframe;    -   backing belt is used to hold material on wheel and mask backside        of material;    -   anode is stationary inside wheel.        Advantages:

Fast, material never stops for selective plating; no timing issues;pumps, rectifiers, and drive system are on continuously; low costbecause system is mechanically uncomplicated.

Disadvantages:

Loose plating boundaries, poor spot location, and potential bleedout arenot critical issues for the present invention.

FIGS. 3A and 3B illustrate schematically important portions of the“Wheel System” apparatus used for fabricating leadframes according tothe invention. FIG. 3A shows the continuous leadframe strip 301 arrivingfrom the nickel pre-plating station, described above, and progressingcontinuously from entry zone 302 through the wheel and masking sectionsof the apparatus to the exit zone 304. The wheel and masking sectionsconsist of the plastic wheel 303 and the rubber masking belt 305. Thetension of the belt is adjustable for precision masking. The plasticwheel 303, shown in more detail in FIG. 3B, provides spot apertures 303a and index pins 303 b. The plating solution is pumped from the centralregion of the wheel system through the anode and sparger 307, sprayingthe solution (the cathode 310 is located in front of entry zone 302).

The relatively thick palladium layers are plated within the wheel andmasking section. One plating section, shown in FIG. 3A, serves the firstsurface of the leadframe, an analogous section, not shown in FIG. 3A,serves the second surface. In contrast, the thin palladium films soimportant for the present invention are plated in entry zone 302 andexit zone 304. The leadframe strip passes through these zones fast andthe concentration of plating solution in the ambient is low.

After exiting from zone 304, the leadframe strip 301 progresses to therinsing and drying stations and further processings steps.

A more precise, but also more costly and slower selective platingtechnique is the step-and-repeat process.

Step and Repeat

-   -   Leadframe material is stopped in selective plating head;    -   rubber mask system clamps on material;    -   plating solution is jetted at material;    -   current is applied;    -   current is shut off;    -   solution is shut off;    -   head opens;    -   material moves.        Advantages:

Very sharp plating spot with excellent edge definition; very good spotlocation capability when used with index holes, pins and feedback visionsystem.

Disadvantages:

Slow; material must stop during selective plating; expensive equipmentto buy and maintain; timing issues; lots of moving parts.

FIGS. 1 and 2 depict the first embodiment of the invention, which isespecially applicable to through-hole leadframes. The spot-platedpalladium is deposited on the first (top) and second (bottom) surfacesof the external lead segment ends. In FIG. 4, the second embodiment ofthe invention is illustrated, especially applicable to surface-mountleadframes. Here, the spot-plated palladium is deposited only on thebottom surface of the external lead segment ends (about 70–80 nm thick),which are involved in the solder attachment process of the semiconductordevice to interconnection boards or motherboards. On the lead surfacesnot involved in solder attachment, palladium may only be 20–30 nm thick.Of course, the palladium is also plated onto the wire bonding areas ofthe internal lead ends. Typically, these external device leads areformed, usually in either gull-wing shape or J-shape. An example for agull-wing shaped device is shown in FIG. 4.

In the schematic cross section of FIG. 4, the copper or copper alloyleadframe 401 of the invention is shown as applied in the assembly of asemiconductor package generally designated 400. Leadframe 401 has a chipmount pad 402 onto which an IC chip 403 is attached using adhesivematerial 404 (typically an epoxy or polyimide which has to undergopolymerization). Leadframe 401 further has a plurality of lead segments405. These lead segments have a first end 405 a near the chip mount pad402 and their second end 405 b remote from mount pad 402.

As shown in FIG. 4 schematically, leadframe 401 comprises base 406 madeof copper or copper alloy. On the surface of this copper is a sequenceof layers, described in detail in FIG. 2. Closest to the copper is afirst layer 407 of nickel. This layer is actually a stack of layers,followed by spot-plated layers 408 and 409 of palladium. Palladium layer609 is incorporated into the meniscus of the bulk solder 610 in theprocess of surface-mounting device 400 onto a substrate or board 420.

In FIG. 4, bonding wires 411 have stitches 412 welded to the palladiumsurface 408 of the first ends 405 a of leadframe segments 405. Thebonding wires are selected from a group consisting of gold, copper,aluminum, and alloys thereof. Any of these metals provide reliable weldsto the layered leadframes of the invention.

As shown in FIG. 4, the second ends 405 b of segments 405 are suitablefor bending and forming due to the ductility of the copper base and theplated nickel layer. Using this malleable characteristic, segments 405may be formed in any shape required for surface mounting or any othertechnique of board attach of the semiconductor devices. The bending ofthe segments does not diminish the corrosion protection of the secondsegment ends 405 b. For example, FIG. 4 indicates a so-called “gull wingshape” of segments 405. This shape is widely used for IC packages in theso-called “small outline” configuration, as illustrated in FIG. 4.

The palladium spot-plated copper leadframe of the invention provides foreasy and reliable solder attachment to boards or other parts of theformed leadframe segments. In FIG. 4, solder attach material 410comprises materials selected from a group consisting of tin/leadmixture, tin/indium, tin/silver, tin/bismuth, tin/copper,tin/silver/copper, and conductive adhesive compounds. All of thesematerials show good wetting characteristics to the plated nickel surfaceof the copper leadframes.

In FIG. 4, molding compound 413 encapsulates the mounted chip 403,bonding wires 411 and the first ends 405 a of the lead segments 405. Thesecond, remote ends 405 b of the segments are not included in the moldedpackage; they remain exposed for solder attachment. Typically, theencapsulation material 413 is selected from a group consisting ofepoxy-based molding compounds suitable for adhesion to the leadframesurfaces. For the thin palladium film of the invention, excellentadhesion characteristics to molding compounds can be achieved,preventing package delamination, moisture ingress and corrosion. Thisimproved adhesion of the molding compound is achieved, according to theinvention, by creating a leadframe surface exhibiting some nickel andnickel oxide in conjunction with palladium.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the material of the semiconductor chip maycomprise silicon, silicon germanium, gallium arsenide, or any othersemiconductor material used in manufacturing. As another example, thedesign, cover area and fabrication method of the palladium layer may bemodified to suit specific leadframe or substrate needs. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

1. A device; comprising: a leadframe including: a base metal structureselected from a group consisting of copper, copper alloy, aluminum,iron-nickel alloy, brass, and invar; a nickel layer covering the basemetal structure; a palladium film covering the nickel layer; a palladiumlayer selectively covering areas of the leadframe suitable for bondingwire attachment and solder attachment; and the palladium layer providingvisual distinction to the areas covered by the layer over the areas ofthe palladium film uncovered by the palladium layer; and a semiconductorchin mounted on the leadframe.
 2. The device of claim 1, in which thenickel layer has a thickness in the range from approximately 1 to 3 μm.3. The device of claim 1, in which the nickel layer is a stack of anickel layer in the thickness range from about 30 to 50 nm, plated ontosaid base metal, followed by a palladium/nickel layer in the thicknessrange from about 30 to 50 nm, followed by a nickel layer in thethickness range from about 1.0 to 3.0 μm.
 4. The device of claim 1, inwhich the palladium film has a thickness from about 1 to 5 nm.
 5. Thedevice of claim 1, in which the palladium layer has a thickness fromabout 70 to 90 nm.
 6. The device of claim 1, in which the palladiumlayer covers selective areas having boundaries of loose tolerance. 7.The device of claim 1, in which the base metal has a thickness betweenabout 100 and 250 μm.
 8. The device of claim 1, in which the solderattachment comprises materials selected from a group consisting oftin/lead, tin/indium, tin/silver, tin/bismuth, tin/copper,tin/silver/copper, and conductive adhesive compounds.
 9. The device ofclaim 1, in which the solder layer has a reflow temperature compatiblewith wire bonding temperatures and molding temperatures.
 10. The deviceof claim 1, in which the leadframe further includes a chip mount pad foran integrated circuit chip and a plurality of lead segments, eachsegment having a first end near said mount pad and a second end remotefrom said mount pad.
 11. The device of claim 10, in which thesemiconductor chip is mounted on the chip mount pad.
 12. The device ofclaim 11, further comprising a bonding wire connecting the chip and afirst end of the lead segments.
 13. The device of claim 12, furthercomprising encapsulation material surrounding the chip, the bondingwire, and the first ends of the lead segments; and leaving the secondends of the lead segments un-encapsulated.
 14. The device of claim 12,in which the bonding wire is selected from a group consisting of gold,copper, aluminum and alloys thereof.
 15. The device of claim 11, inwhich the bonding wire contacts the first end with a stitch bond, a ballbond, or a wedge bond.
 16. A method for fabricating a semiconductordevice; comprising the steps of: providing a leadframe, which includes:a base metal structure; a nickel layer covering the base metalstructure; a palladium film covering the nickel layer; a palladium layerselectively covering areas of the leadframe suitable for bonding wireattachment and solder attachment; and the palladium layer providingvisual distinction to the areas covered by the layer over the areas ofthe palladium film uncovered by the palladium layer; and mounting asemiconductor chip on the leadframe.